Originally Posted by Spoakland

Depends on what experience you want to improve on your bike. If it's just a matter of a better fit, I agree with the earlier post that getting yourself fitted by a specialist is the way to go. They'll help you with seat height/positioning which may or may not impact your reach. You can buy a shorter stem or get one with a higher angle (8/15/24/30 degree, etc.) or you just may need to move it up a spacer or two.

If you're looking for a faster ride, thinner tires are the way to go. I went from 32s to 28s on my hybrid, and now I have 23s on my road bike and it's like night and day. You could replace the wheels themselves, but depending on how much you want to spend, you may be talking yourself into a new bike. Definitely look for good sales if that's where you're headed. Do you have clipless pedals? That's another area that makes a big difference.

I agree with others -- seat post and, to a lesser extent handlebars, are the last things I'd spend money to upgrade. Little utility for a lot of money IMHO.